168 MAZATLAN BIVALVES 



Shell extremely thin ; lower valve white, smooth ; upper 

 whitish, more or less stained with golden yellow, rarely salmon 

 coloured, extremely rarely greenish, most rarely with a slight 

 touch of pink ; smooth, generally developing irregular radia- 

 ting ribs near the margin, sometimes strong ones all over. 

 Occasionally concentric waves of growth are developed. Lower 

 valve dead white within near the somewhat large (sometimes 

 small) hole, gradually becoming transparent; upper valve 

 with a denned white spathula, somewhat in the shape of an 

 isosceles ^triangle with an excurved base, radiating from the 

 light green ligament, and enclosing the scars. The plug scar 

 is not much larger than the others, and is rounded subquadrate. 

 The others are round, slightly separated, and at a variable 

 angle according to the shape and age of the shell. In different 

 specimens examined, the lines joining the centres of the ter- 

 minal scars with the centre of the middle one, are inclined at 

 angles varying from 105 133 ; shewing how necessary it is 

 to be cautious in founding species merely on the position of 

 the scars. If this divergence of central angle were given in 

 descriptions, it would greatly aid the student. The subcardi- 

 nal scar is small and very slightly impressed. There is a short 

 support from the hinge fulcrum in the lower valve. The plugs, 

 of which a few were fortunately preserved on the outer valves, 

 are very different in appearance from those of A. ephippium ; 

 being large and spreading at the base, but short and small in 

 the attached portion. Extremely fine lines run down the raised 

 part to the margin of the base , but the rest of the base is not 

 grooved. As the shell grows older, it appears to increase the 

 raised part near the umbo, withdrawing from the grooved part 

 of the base, which is no longer covered by the hole. The hole 

 is very much larger than the raised part, which fills only the 

 middle of it. One plug has a large tail, like the expanded foot 

 of a Cyclas, from the gradual shifting of the animal. The raised 

 part has generally a horse-shoe extremity, sometimes built up 

 with very loose shelly matter. The plug under the microscope 

 appears essentially shelly, though with much animal matter. 

 Very young shells are generally orbicular, beginning life as an 

 irregular body without prominent apex, shaped like Discina. 

 The smallest valve is '09 across ; the largest specimen measures 

 long.1'9, lat.2'^, alt. * 14 



Hob. California Lady Wigram, B. M. Monterey, 60 fm., Ma- 

 jor Rick. La Paz, do. Panama ; at low water mark ; 1 sp. : 

 C, B. Adams. Mazatlan : common and very fine ; L'pool Col, 



